Seeded Raman amplifier in a linear configuration for generating a 1240 nm laser

ABSTRACT

A third-order Stokes wavelength seed signal at the desired output wavelength of 1240 nm and a zeroth-order Stokes wavelength signal at 1066 nm are input into a Raman amplifier comprised of two Raman resonators in a linear configuration. The first resonator converts the zeroth-order Stokes wavelength signal at 1066 nm into a first-order Stokes wavelength signal at 1118 nm, and also outputs the third-order Stokes wavelength seed signal at 1240 nm. The second resonator then converts the 1118 nm output from the first resonator into a second-order Stokes wavelength signal at 1176 nm, which amplifies the 1240 nm seed signal power level. Each Raman resonator includes a photosensitive Raman fiber communicating with a plurality of high-reflector Bragg gratings. The linewidths of the second and third Stokes orders are controlled by adjusting the resonant bandwidth of the second Raman resonator by offsetting the respective center wavelengths of the high-reflector Bragg gratings.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of pending U.S. patent application Ser.No. 14/732,723, filed on Jun. 6, 2015, which is a division of U.S. Pat.No. 9,054,499, filed on Jun. 23, 2014 and issued on Jun. 9, 2015, whichis a division of U.S. Pat. No. 8,761,210, filed on Jun. 13, 2013 andissued on Jun. 24, 2014, which is a division of U.S. Pat. No. 8,472,486,filed on Aug. 17, 2011 and issued on Jun. 25, 2013, and claims thebenefit of the foregoing filing date.

STATEMENT OF GOVERNMENT INTEREST

The conditions under which this invention was made are such as toentitle the Government of the United States under paragraph 1(a) ofExecutive Order 10096, as represented by the Secretary of the Air Force,to the entire right, title and interest therein, including foreignrights.

FIELD OF THE INVENTION

The invention relates generally to high-power Raman amplifier systemshaving linewidths from narrow to broad, functioning in the near-infraredspectral region for numerous applications. Such applications include anarrow linewidth 1178 nm sodium guidestar laser for improved spacesituational awareness, a 1240 nm source for remote sensing of water, and1300-1500 nm lasers for telecommunications.

BACKGROUND OF THE INVENTION

In general, there is a lack of efficient, high-power lasers in the1100-1500 nm region with a controllable linewidth. Lasers in the1100-1500 nm spectral region are difficult to obtain, since manymaterials don't lase in this region and those that do lase in parts ofthe spectral region such as bismuth co-doped silica or Yb-doped silica,do so inefficiently. One way of obtaining photons in this spectralregion is through the nonlinear process of Stimulated Raman Scatteringwhich acts to shift the initial pump wavelength out to longerwavelengths. This process, which occurs at high optical intensities,involves the coupling of light propagating through the non-linear mediumto the vibrational modes of the medium. The result is re-radiated lightwhich is shifted to a different wavelength. Light upshifted inwavelength is commonly referred to as a Stokes line, whereas lightdownshifted in wavelength is referred to as an anti-Stokes line.To-date, a controllable linewidth, high-power Raman laser with outputpowers approaching 100 W has not been reported.

The typical Raman amplifier tends to be in several forms. The first formis seeded with an initial pump signal of relatively low power (zerothorder Stokes line) that is free-space coupled or spliced into thesystem. Multiple orders of Stokes lines are then created in one or moreRaman resonators. Each Raman resonator in the system is defined by apair of Bragg gratings centered at the wavelength of the Stokes orderinvolved. The output of one Raman resonator is injected into the Ramanresonator centered at the next highest Stokes order. The highest orderStokes line generated is the output of the laser. Such amplifierstypically tend to have low Raman conversion efficiencies and low outputpowers (<5 W) due to the relatively low intensity of injected pumpsignal in the core. Raman fibers hundreds of meters in length are oftenrequired to enable adequate conversion of the pump signal to a longerwavelength. Such lasers also tend to have broad linewidths, since theRaman process is initiated by broad-band spontaneous Raman scatteringwithin the fiber. It has also been observed that, as the power in thesystem is increased and/or higher-order Stokes lines are generated, thelinewidth of the output tends to further broaden. To conclude, this typeof Raman laser tends to have low output powers in addition tolinewidths, which are not controlled and are broad.

A variant of the above are all-fiber Raman systems where the pump iseither generated or amplified by a rare-earth amplifier that is spliceddirectly onto the Raman resonators (Nicholson, US Patent Application No.2010/0284060A1 and Nicholson, et al., Optics Letters 35(18)(2010) 3069).This type of system has the potential of generating high output powers(>50 W) because of direct injection of high power levels of the zerothorder Stokes directly into the core of the fiber via an amplifier.Highest power levels were achieved by Nicholson et al., who demonstrated81 W of output power at 1480 nm when 162 W of 1117 nm pump from anamplifier was injected into 120 m of Raman filter fiber. Although highpower levels were achieved, a long length of Raman fiber was stillrequired in order to enable sufficient buildup in the Raman cavities ofthe various Stokes lines from spontaneous Raman scattering. Theresultant linewidth of these lasers is broad, since no measures aretaken to control linewidth broadening. To conclude, this kind of laseris capable of high output powers but the linewidths tend to be broad.

Another variant of a Raman amplifier involves both seeding with thedesired output signal (N^(th) order Stokes) and a pump signal (zerothorder Stokes) through either a wavelength division multiplexer (WDM) oran optical circulator (see for example, Taylor et al., US PatentApplication No. 2011/0038035A1). In such systems, one or more stages ofRaman amplification may be necessary in order to generate the N−1^(th)order Stokes signal necessary for amplification of the N^(th) orderStokes seed. Because both WDMs and optical circulators are powerlimited, the amount of pump signal (zeroth order Stokes) and desiredoutput signal (N^(th) order Stokes) that can be fed into the system islimited. Because of low levels of pump signal in the system, outputpower levels are limited, efficiencies tend to be low, and extremelylong Raman fibers (100 m or more) are necessary. Output power levels arealso limited by Stimulated Brillouin Scattering for narrow linewidthsignals due to the long Raman fiber. Relative to an unseeded system, thelinewidth of the amplified output signal is controllable to a certaindegree, since the seed signal will dominate the spontaneous Ramanscattering. Even so, linewidth broadening will still occur because offour-wave mixing. To conclude, this sort of system is capable of loweroutput powers with some control of the linewidth.

Another variant is to seed a system with power from a rare-earth-dopedoscillator that is spliced directly onto a Raman resonator (Mead, USPatent Application No. 2011/0122482). In this patent application, themain focus is on generating closely spaced wavelengths from multipleRaman fiber amplifiers through stretching of fiber to enable spectralbeam combination of eye-safe lasers. One embodiment is shown where thesystem is seeded with the desired output wavelength (N^(th) orderStokes) and is Q-switched to generate pulsed light. No continuous waveconfiguration is discussed. The rare-earth-doped oscillator, which isunseeded, is co- and counter-pumped with diodes. The output wavelengthof the oscillator is determined by the Bragg gratings that form thecavity and will have a broad linewidth, since it is seeded withamplified spontaneous emission. The rare-earth-doped oscillator/Ramanresonator described in this embodiment should be capable of high outputpower because of direct injection of the zeroth order Stokes from therare-earth-doped oscillator into the core of the fiber in the Ramanresonators but, once again, the N^(th) order Stokes seed will experiencelinewidth broadening as it is amplified since no measures are taken tomitigate four-wave mixing. This configuration will also be power limitedrelative to what would be achievable using a comparable rare-earth-dopedamplifier because of thermal issues associated with power buildup in thecavity of the rare-earth-doped oscillator. Also, for high-powerapplications, fully nested Raman cavities as they appear in this patentapplication will experience higher thermal stresses than a lessoverlapped configuration. In addition, the embodiment in this patentapplication may experience damage upstream, since no measures are takento mitigate light leaking out of the gratings and propagating backwards.To conclude, the system described will be capable of high-power outputpulses (but not continuous wave operation), but because no measures aretaken to control four-wave mixing, broadening of the signal linewidthwill occur. Also, high power levels within the rare-earth-dopedoscillator and the Raman cavities may be a limiting issue.

An important narrow-linewidth application for Raman lasers is thegeneration of 1178 nm for guide star (e.g., sodium guidestar) laserapplications. This is important since the resolution of terrestrialtelescopes is limited by wave front distortion caused by atmosphericturbulence. This distortion can largely be overcome by the use ofadaptive optics in which the surface of a deformable telescope mirror isvaried as a function of time to compensate for atmospheric turbulencethrough which light from distant objects must travel. Measuring thedistortion requires that there be a bright optical source in the sky,such as a bright star, located close to the object to be observed. Sincebright stars are infrequently located close to objects of interest. Analternative is to energize a layer of sodium atoms which is naturallypresent in the mesosphere at an altitude of around 90 kilometers. Thesodium atoms then re-emit the laser light, producing a glowingartificial star whose radiation can provide a wavefront reference toenable correction of the image for atmospheric induced aberrations.

The sodium guidestar laser application is very challenging in thatoutput powers on the order of 50 W of 589.15908 nm on the sodium D_(2a)line with a linewidth of 10 MHz is required. For two-line systems, anadditional 10 W of 589.15709 nm on the sodium D2B line with a linewidthof 10 MHz is desired. One very successful method for generating 50 W of10 MHz linewidth 589 nm involves the use of traditional rod lasertechnology and sum-frequency generation via a nonlinear crystal. Astate-of-the-art system developed at the Air Force Research Laboratory(Denman, et al., U.S. Pat. No. 7,035,297) contains 1064 and 1319 nmresonant cavities in addition to a doubly resonant sum-frequencygeneration cavity containing a lithium triborate crystal. To maintainlock on the D_(2a) line of sodium, multiple Pound-Drever-Hall lockingloops are utilized. A maximum of 50 W of 589 nm was achieved by thissystem. The major drawback associated with this system is its size andcomplexity.

The most successful attempt at addressing the requirement for the sodiumguidestar laser using fiber was accomplished by the European SouthernObservatory (ESO) (Feng, et al., Optics Express 17(21)(2009) 19021). Inthis concept, a Raman amplifier is directly core pumped with 1121 nm ina counter-pumped configuration while being seeded with narrow linewidth1178 nm through a wavelength division multiplexer (WDM). The amount ofnarrow linewidth 1178 nm that can be generated was limited to less than39 W. Coherent beam combination has been used to generate 26 W of 589 nmfrom two 1178 nm sources and greater than 50 W from three Raman fiberamplifiers. The linewidth of the 589 nm was found to be less than 2.3MHz. The bottleneck associated with this technique is a power limitationassociated with the WDM that clamps the amount of 1121 nm that can beinjected into the system. This, coupled with problems from SBS resultingfrom a Raman fiber of length 150 meters, results in relatively lowoutput powers of 1178 nm. As a result, in order to generate the levelsof 589 nm desired by the various telescopes, coherent beam combinationof multiple Raman amplifiers is necessary. The result is a system ofincreased complexity.

Since astronomical telescopes are located in remote sites and oftenoperate under difficult conditions, it is desirable to use a compact,maintenance-free, and rugged laser for the guide star system, such as afiber laser. Because there are no fiber gain media that lase directly at589 nm, the most promising way, to date, to achieve this is throughsecond harmonic generation of 1178 nm. The present invention has thepotential to generate 100 W of narrow linewidth 1178 nm from anall-fiber Raman laser for frequency doubling to 589 nm on the D_(2a)line.

Another exemplary application in the 1200-1300 nm region is remotesensing of the water content of vegetation on earth from space. The goalis to improve the understanding of the biophysical and ecologicalprocesses governing the linked exchanges of water, energy, carbon andtrace gases between the terrestrial biosphere and the atmosphere byimproving satellite data products for models. 1240 nm having linewidthson the order of 100 MHz to 1 GHz in conjunction with 858.5 nm is ofinterest since the optical index R858.5/R1240 is mainly driven by thewater thickness with smaller effects due to cellulose, lignin, andprotein variation. The wavelength 1240 nm has been generated in the pastusing several methods to include external cavity diamond Raman lasers,an intra-cavity Raman laser, GaInNAs semiconductor diode lasers,optically pumped GaInNAs/GaAs and a Cr:forsterite multi-terawattamplifier laser. Output powers are typically less than 5-6 W with thelinewidth being determined by properties of the resonating cavity. Highpower levels of 1240 nm can be achieved in an all-fiber system with acontrollable linewidth by the present invention.

Another exemplary application in the 1300-1500 nm region is theexpansion of telecommunications bandwidth into the O, E, and S bands.This will require multiple closely spaced lasers having linewidths wellexceeding a GHz in this spectral region. At the present time, some workhas been done with seeded Raman lasers at 1300 nm. Such lasers areinefficient and typically of low output power because of having toinsert and remove light through power sensitive components. UnseededRaman lasers using phosphosilicate fiber by itself and in conjunctionwith germanosilicate fiber have been used to generate light in the1400-1500 nm region. Such lasers can be of rather high output power butare of broad linewidth because of the lack of a seed or other measuresto control the linewidth. The invention in this application can beutilized to provide a series of lasers having controllable linewidths inthe 1300-1500 nm region to enable expansion of the telecommunicationsindustry into other bands.

The current invention is aimed at overcoming the shortcomings associatedwith the current state-of-the-art to obtain high power (>50 W) from aRaman amplifier while controlling linewidth broadening.

SUMMARY

The current invention involves a Raman amplifier having a novel designenabling high Raman conversion efficiencies and output powers inaddition to linewidths, which can be controllable by a seed source. Inthis invention, a rare-earth-doped Raman amplifier can be spliceddirectly onto a Raman resonator system. The rare-earth-doped amplifiercan be both seeded with the initial signal (zeroth order Stokes) and thedesired output signal (N^(th) order Stokes) through a wavelengthdivision multiplexer (WDM). The linewidth of the N^(th) order Stokesseed can be equivalent to the desired output wavelength. The linewidthof the zeroth order Stokes can be broad enough to prevent StimulatedBrillouin Scattering (SBS) from being a limiting factor. The nonlinearprocess of SBS can thereby be avoided for all except possibly the N^(th)Stokes signal if it is of narrow linewidth. Because of power limitationsassociated with the WDM, it is important to amplify the initial signal(zeroth order Stokes) to the desired level via an amplifier downstreamfrom the WDM. This amplifier may consist of one or multiple stages witheach stage being pumped with diodes. The desired output signal andamplified initial signal can then both be injected into the Ramanresonator(s) where multiple orders of Stokes, up to the N−1^(th) Stokesorder, are generated in one or more Raman amplifiers. Note that theRaman Resonator system can be configured to be fully nested orcompletely unnested. The desired signal (N^(th) order Stokes) passesthrough the system and is amplified by the N−1^(th) order Stokes signal.To increase the efficiency of the system, a highly reflective Bragggrating centered at the pump wavelength can be included downstream fromthe Raman fiber to enable reflection of any unused pump light backthrough the Raman fiber. In addition, long period gratings or tiltedBragg gratings can be placed between the rare-earth-doped Ramanamplifier and the Raman resonators to protect all of the componentsupstream as well as to ensure high efficiency of the rare-earth-dopedamplifier at the zeroth order Stokes wavelength. The linewidth can becontrolled, first by seeding and subsequently through a slightwavelength offset of one Bragg grating in each resonator to limit thelinewidth of the Stokes order that resonates as well as by providingsome mitigation of four-wave mixing, the primary cause of linewidthbroadening. This offset can be accomplished in various ways to includetemperature, bending, stretching, etc. In addition, high-dispersionRaman fiber can also be utilized to break the phase matching conditionfor four-wave mixing.

Relative to the current state-of-the-art, the present invention iscapable of high-power operation with a controllable linewidth. Becausethe rare-earth amplifier can be spliced directly onto the Ramanresonator, high-power operation is possible, since there are nobottlenecks involving components, which are power limited. Also, becausea rare-earth amplifier can be used instead of an oscillator which buildsup high power levels in the cavity, the maximum output power achievablefrom the rare-earth amplifier and ultimately the Raman resonator systemcan be greater than that achievable with a rare-earth oscillator. Also,because a high power level of the N−1^(th) Stokes order required foramplification of the desired output wavelength can be obtained withinthe Raman resonator system, high Raman conversion efficiencies are alsoobtainable. This enables usage of a very short length of Raman fiber (acouple of meters), which in turn leads to a high threshold forStimulated Brillouin Scattering and higher, achievable output powers fornarrow linewidth outputs. In addition, a controllable linewidth is alsopossible due to various linewidth broadening mitigation measures toinclude an offset of the Bragg grating pairs associated within eachresonator, utilization of high dispersion Raman fiber, and utilizationof a signal seed with the desired linewidth. All linewidths within thesystem except the seed should be broad enough to avoid SBS issues. Thecurrent invention is also compatible with all techniques to mitigateSBS, including multiple temperature zones, stress, acoustically tailoredfiber, different types of fiber, large mode area fiber, etc. Finally,the invention is compatible with both phase modulation (PM) and non-PMmodes of operation, as well as pulsed or continuous wave operation.

An exemplary application is the generation of very narrow linewidth 1178nm for sodium guidestar laser applications. The present inventiondetails a method to obtain high output powers of 100 W or more from an1178 nm laser with a linewidth of less than 10 MHz in an all-fiber Ramansystem. This application involves a system that is co-seeded into thefiber core with both a narrow linewidth (10 MHz) 1178 nm signal (2^(nd)order Stokes) and a 1069 nm signal (zeroth order Stokes) that is broadenough that SBS will not be an issue.

Another exemplary application in the 1200-1300 nm region is remotesensing of the water content of vegetation on earth from space. 1240 nmwith a controllable linewidth can be achieved in a seeded system usingtwo Raman cavities along with germanosilicate fiber. A final exemplaryapplication of the invention in this patent holds the potential forproducing high power output power from an all fiber system between1300-1500 nm with a controllable bandwidth through the usage of multipleRaman resonator cavities.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a generic rare-earth-doped amplifier/Ramanamplifier in a fully nested configuration;

FIG. 2 is a diagram of a generic rare-earth-doped amplifier/Ramanamplifier in a linear (non-nested) configuration;

FIG. 3 is a diagram of a sodium guidestar laser system configuration;

FIG. 4 is a plot of the 1069 nm and 1178 nm cavity intensity as afunction of the position in the fiber for the case of 9 meters of 20/400fiber pumped with 59.3 W of 1069 nm and seeded with 3 W of 1178 nm forthe sodium guidestar laser configuration of FIG. 3;

FIG. 5 is a plot of the first Stokes line (1121 nm) cavity intensity inthe forward and backward directions as a function of the position in thefiber for the case of 9 m of 20/400 fiber pumped with 59.3 W of 1069 nmand seeded with 3 W of 1178 nm;

FIG. 6 is a plot of the calculated reflectivity for a pair of uniformgratings 5 cm in length with B=10 that are offset by 0.02 nm. Theresulting cavity has high reflectivity only over a few GHz with thesegratings;

FIG. 7 is a plot of the effect of 1069 nm seed power on the 1121 nmcirculating power and the relative 1178 nm output power;

FIG. 8 is a plot showing the effect of 1178 nm seed power on the 1121 nmcirculating power and the 1178 nm relative output power; and

FIG. 9 is a plot showing the effect of the 1121 nm resonator length onthe 1121 nm circulating power and the 1178 nm relative output power.

FIG. 10 is a diagram of a 1240 nm rare-earth-doped/Raman amplifier in afully nested configuration.

FIG. 11 is a diagram of a 1240 nm rare-earth-doped/Raman amplifier in alinear (non-nested) configuration.

FIG. 12 is a generic diagram of a 1300-1500 nm rare-earth-doped/Ramanamplifier.

DETAILED DESCRIPTION

The two generic embodiments representing the extremes of Raman resonatorconfigurations for the invention described in this patent are described.One embodiment consists of a rare-earth-doped fiber amplifier spliced toa Raman resonator configuration that is fully nested. The otherembodiment consists of a rare-earth-doped fiber amplifier spliced to acompletely linear, unnested, Raman resonator configuration. Anyoneskilled in the art will realize that other configurations lying betweenthese two extremes are possible.

Nested Raman Resonator Embodiment

A block diagram of the fully nested Raman resonator embodiment is shownin FIG. 1. The first device in the diagram is a seed source 1 tuned tothe desired output wavelength, the N^(th) order Stokes line. It may becreated in a number of ways to include frequency doubling of a source ata longer wavelength. This device needs to have the linewidth desired ofthe output. Another device in the diagram is a seed source 2 tuned tothe necessary wavelength (zeroth order Stokes line) to enable generationof an N^(th) order Stokes signal at the desired wavelength. This deviceneeds to have a linewidth broad enough so as to prevent SBS from beingan issue. The output of the source tuned to the N^(th) order Stokes line1 is passed through an isolator 3 and into a WDM 5. The output of thesource tuned to the zeroth order Stokes 2 is passed through an isolator4 and into the WDM 5. The WDM 5 puts both the zeroth order and theN^(th) order Stokes signals into the core of the fiber. Both signalsthen enter a rare-earth-doped amplifier 6 which amplifies the signal.The amplifier 6 may consist of one or more stages in order to reach thedesired power level of the zeroth order Stokes signal. The amplifiedzeroth order Stokes signal and the seed for the N^(th) order Stokes lineis injected into Raman resonators in a fully nested configuration asdefined by pairs of Bragg gratings 8-13. Prior to entering the Ramanresonators, the zeroth and N^(th) order Stokes signals pass throughseveral long period or tilted Bragg gratings (LP/TB) 14-16 whose role isto bring about absorption of any first through N−1^(th) order Stokessignals that leak through the Bragg gratings 8-10 back toward therare-earth-doped amplifier 6. The Raman resonators are nested such thatthe innermost resonator is tuned to the first order Stokes line withBragg gratings 10 and 11 both being high reflectors with a centerwavelength at the first order Stokes line. The next innermost resonatoris tuned to the second order Stokes line etc. via gratings 9 and 12which are high reflectors centered at the second order Stokes line. Theoutermost Raman resonator is tuned to the N−1^(th) order Stokes line viagratings 8 and 13 which are high reflectors centered at the N−1^(th)order Stokes line. These nested Raman resonators 21 are enclosed bydashed lines in FIG. 1. The zeroth order Stokes signal passes into thenested Raman resonators and is converted to the first order Stokessignal by the resonator tuned to the first order Stokes line 10, 11. Anyunconverted zeroth order Stokes signal is then reflected back into theRaman fiber 17 via a high reflector 7 tuned to the zeroth order Stokesline. The first order Stokes signal is then converted to the secondorder Stokes signal by the resonator tuned to the second order Stokesline 9, 12 and is eventually converted to the N−1^(th) order Stokessignal by the resonator tuned to the N−1^(th) order Stokes line 8, 13.Because there are no highly reflective Bragg gratings with a centerwavelength at the N^(th) order Stokes line, the N^(th) order Stokessignal passes straight through the nested Raman resonators defined bythe Bragg gratings 8-13 and is amplified by the N−1^(th) order Stokessignal in the Raman resonator tuned to the N−1^(th) order Stokes line.The amplification occurs in the Raman fiber 17 which is eitherphosphosilicate, germanosilicate or some other variety of photosensitivefiber. The output of the amplifier is the N^(th) order Stokes signal 20.Because of potentially low seed powers at the desired output wavelength,it may be necessary to have multiple rare-earth-doped amplifier/Ramanresonator configurations in series to enable amplification of thedesired output wavelength to the power levels desired. This embodimentis compatible with all SBS mitigation techniques to include multipletemperature zones, strain, large mode area fibers, multiple fiber types,acoustically tailored fiber, etc. In addition, the embodiment iscompatible with both polarized and unpolarized modes of operation aswell as continuous wave and pulsed operation.

To enable controlled linewidth operation, multiple measures can betaken. These measures include usage of a seed for the N^(th) orderStokes signal having the desired output linewidth. In order to preventthe linewidth of the N^(th) order Stokes seed from broadening as it isamplified, it is imperative that the linewidth of the N−1^(th) orderStokes signal be controlled. This can be accomplished by using alinewidth for the zeroth order Stokes signal that is broad enough toprevent SBS from being an issue. Although seeding the amplifier with thedesired output wavelength will result in a narrower output signal thanif the system were seeded by spontaneous Raman scattering, additionalmeasures are needed within the individual Raman resonators to preventlinewidth broadening of the zeroth order Stokes signal as it isconverted to higher Raman orders. Control of the linewidth of thevarious Stokes orders can be accomplished by shifting the centerwavelength of one grating of each pair 18 slightly through eitherheating, bending, or stretching, e.g., through a grating controller 19.This has the effect of impeding four-wave mixing, the primary source oflinewidth broadening, as well as narrowing the bandwidth that will beamplified at each Stokes order. In addition, high dispersion Raman fibercan be used to break the phase matching condition for four-wave mixing.One or more of these measures in combination should work to limitbroadening of the linewidth.

The factors enabling a high output power level of the N^(th) orderStokes signal in this invention are the generation of very high levelsof the N−1^(th) order Stokes signal in the Raman resonator tuned to theN−1^(th) order Stokes line in addition to seeding with the N^(th) orderStokes signal. This occurs for two reasons, first, high power levels ofthe zeroth order Stokes are directly injected into the system andsecond, because the Raman cavities are defined by high reflector Bragggratings, the power level in the system is able to build up to highlevels in all the Raman cavities. High power levels of the N−1^(th)order Stokes signal which is created from the zeroth order Stokes signalin addition to seeding at the N^(th) order Stokes line leads to goodconversion from the zeroth order Stokes signal to the N^(th) orderStokes signal in a short Raman fiber. In addition, the short Raman fiberenables a high SBS threshold, which enables a higher output power levelof the N^(th) order Stokes signal. Negatively, this embodiment willsuffer substantial thermal stress because of high power levels in theRaman fiber and on the Bragg gratings, since all of the Raman resonatorsoverlap.

Unnested Raman Resonator Embodiment

The embodiment representing the other extreme is a series of completelyunnested Raman resonators. In this configuration, multiple Ramanresonators in series are used to shift the zeroth order Stoke inputsignal out to the N−1^(th) order Stokes line to enable amplification ofthe N^(th) order Stokes input signal. The amplifier is in a linearconfiguration with each Raman resonator having the same or differentkinds of photosensitive (or nonphotosensitive) fiber. The system isco-seeded with both the initial zeroth order Stokes signal as well asthe desired N^(th) order Stokes output signal.

A block diagram of the unnested Raman resonator system is shown in FIG.2. The first device in the diagram is a source 30 at the N^(th) orderStokes line having the desired output linewidth. The source at theN^(th) order Stokes line may be created in a number of ways to includefrequency doubling of a source at a longer wavelength. Another device isa source 31 tuned to the wavelength of the zeroth order Stokes line.This device should have a linewidth broad enough so that SBS is not anissue. The output of the source tuned to the N^(th) order Stokes line 30is passed through an isolator 32 and into a WDM 34. The output of thesource tuned to the zeroth-order Stokes line 31 is passed through anisolator 33 and into the WDM 34. The WDM 34 puts both the zeroth and theN^(th) order Stokes lines into the core of the fiber. The zeroth and theN^(th) order Stokes lines then enter an amplifier 35 which amplifies thezeroth order Stokes signal. The amplifier 35 may consist of one or morestages in order to reach the desired power level for the zeroth-orderStokes signal. The output of amplifier 35 which includes the amplifiedzeroth order Stokes signal and the seed for the N^(th) order Stokes lineis passed through either a long period or tilted Bragg (LP/TB) gratingtuned to the first-order Stokes line 39 and is injected into the firstRaman resonator in a series of Raman resonators within the dashed linesof FIG. 2. The zeroth-order Stokes signal passes through a highreflector Bragg grating 37 tuned to the first-order Stokes line and intothe Raman fiber 48 which may be germanosilicate, phosphosilicate or someother kind of photosensitive (or nonphotosensitive) fiber. Any unusedzeroth-order Stokes signal is reflected back into the Raman fiber 48 bya high reflector Bragg grating 36 set at the zeroth-order Stokes line.In this resonator defined by the high reflector Bragg grating 37 and theBragg grating 38, which is an output coupler (OC), the zeroth orderStokes signal is converted to the first-order Stokes signal via theRaman process in the Raman fiber 48. The size of the Raman shift isdependent on the type of Raman fiber used. Any first-order Stokes signalthat leaks out of the Bragg grating 37 back toward the rare-earth-dopedamplifier 35 will be put into the cladding by the long period or tiltedBragg grating 39, where it will be absorbed. The N^(th) and first-orderStokes signals then pass through a long period grating 43 set at thesecond-order Stokes line and enter the second Raman resonator in theseries defined by the Bragg grating 41, which is a high reflector, andthe Bragg grating 42, which is an output coupler. In this resonator thefirst-order Stokes signal is converted to the second-order Stokes signalin the Raman fiber 49, which may be the same or different than the Ramanfiber 48. Any unused first-order Stokes signal will be reflected backinto the Raman fiber 49 by the high reflector Bragg grating 40 which iscentered at the first order Stokes line. Any second order Stokes signalwhich leaks out of the Bragg grating 41 back toward the rare-earth-dopedamplifier 35 will be put into the cladding by the long period or tiltedBragg grating 43 and absorbed. Eventually, the N−2^(th) order Stokessignal which has been created from the zeroth order Stokes signaltogether with the N^(th) order Stokes signal enters the last Ramanresonator in the series. This resonator is tuned to the N−1^(th) orderStokes line and is defined by the Bragg gratings 45 and 46, which areboth high reflectors. Within this Raman resonator the N−2^(th) orderStokes signal is converted to the N−1^(th) order Stokes signal in theRaman fiber 50, which may be the same or different than the Raman fibers48 or 49. Any unused N−2^(th) order Stokes signal will be reflected backinto the Raman fiber 50 by the Bragg grating 44, which is a highreflector tuned to the N−2^(th) order Stokes line. Also, any N−1^(th)order Stokes signal which leaks out of the Bragg grating 45 back towardthe rare-earth-doped amplifier 35 will be put in the cladding by thelong period or tilted Bragg grating 47 and absorbed. Upon entering theRaman resonator tuned to the N−1^(th) order Stokes line, the seed forthe N^(th) order Stokes line is amplified in a single pass through theresonator. The output of the Raman amplifier is the greatly amplifiedN^(th) order Stokes line 54. Once again, because of potentially low seedpowers at the desired output wavelength, it may be necessary to havemultiple rare-earth-doped amplifier/Raman resonator configurations inseries to enable amplification of the desired output wavelength to thepower levels desired. This embodiment is compatible with all SBSmitigation techniques to include multiple temperature zones, strain,large mode area fibers, acoustically tailored fibers, different types offiber etc. The embodiment is also compatible with both polarized andunpolarized modes of operation as well as continuous wave or pulsedoperation.

To enable controlled linewidth operation, multiple measures can betaken. These include usage of a seed at the N^(th) order Stokes linehaving the desired output linewidth. In order to prevent the linewidthof the N^(th) order Stokes seed from broadening, it is imperative thatthe linewidth of the N−1^(th) order Stokes signal amplifying it beprevented from being too broad. This can be accomplished by using alinewidth for the zeroth order Stokes signal that is broader than thelinewidth required to prevent SBS from being an issue. Although seedingthe amplifier with the desired output wavelength will result in anarrower output signal than if the system were seeded by spontaneousRaman scattering, additional measures are needed within the individualRaman resonators to prevent linewidth broadening of the zeroth-orderStokes signal as it is converted to higher Raman orders. Control of thelinewidth of the various Stokes orders can be accomplished by shiftingthe center wavelength of one grating of each pair 38, 42, and 46slightly by either heating, bending, or stretching, e.g., through thegrating controllers 51-53. This will impede four-wave mixing and willhelp control the bandwidth being amplified at each Stokes order, thushelping to keep the linewidth of the intermediate Stokes orders frombroadening. Also, usage of a high-dispersion Raman fiber to break thephase-matching condition associated with four-wave mixing is anothermeasure that can be taken. One or more of these measures in combinationshould work to control the linewidth of the output of the Ramanamplifier.

As before, the factors enabling a high output power level of the N^(th)order Stokes signal in this invention are the generation of high levelsof the N−1^(th) order Stokes signal in the Raman resonator tuned to theN−1^(th) order Stokes line in addition to seeding with the N^(th) orderStokes signal. Relative to the previous embodiment where the Bragggratings defining the Raman cavities were high reflectors, power levelsobtainable in the resonators in this system will be less since one Bragggrating defining each cavity will be an output coupler. In addition, thepower level in each successive Raman cavity will diminish. Because ofthis, a decreased output power level of the N^(th) order Stokes signalrelative to the fully nested configuration is expected. Positively, thisembodiment relative to the fully nested configuration will have muchless thermal stress associated with it since there will be less power ineach Raman fiber and on each Bragg grating.

Sodium Guidestar Laser System

An exemplary application of the invention is a system that generates anarrow linewidth laser beam of about 1178 nm for second-harmonicgeneration to 589 nm for guide star lasers (e.g., sodium guidestarlasers). The main requirements associated with a guide star laser (whichwill be referred to herein as “sodium guidestar laser”) system are:linewidths on the order of 10 MHz along with output powers of 589 nm onD_(2a) greater than 50 W. A linewidth of 10 MHz which equates to thenatural linewidth of sodium is required in order to enable excitation ofthe same velocity group. The invention described in this application hassufficient output power at 1178 nm to enable the 589 nm target powerlevels.

A block diagram of this system is shown in FIG. 3. The first device inthis diagram is the second-order Stokes signal 60 at 1178 nm, thedesired output signal, having a narrow linewidth (10 MHz). The system isco-seeded with a zeroth-order Stokes seed 61 at 1069 nm having alinewidth broad enough so that SBS is not an issue. The zeroth-orderStokes signal at 1069 nm is passed through an isolator 63 and into a WDM64. The second-order Stokes signal at 1178 nm is passed through anisolator 62 and into the same WDM 64. The WDM 64 places both the zerothand the second-order Stokes signals into the core of the fiber. Both thezeroth and second-order Stokes signals enter an ytterbium-dopedrare-earth amplifier 65 where the zeroth-order Stokes signal isamplified. The amplified zeroth-order Stokes signal along with thesecond-order Stokes signal pass through a long-period or tilted Bragg(LP/TB) grating 69 tuned to 1121 nm and enter the Raman resonatorconfiguration defined by high-reflector (HR) Bragg gratings 67 and 68tuned to 1121 nm. The zeroth-order Stokes 1069 nm signal, upon enteringthe 1121 nm Raman resonator, is converted to the first-order Stokessignal at 1121 nm by the Raman process in the silica or germanosilicatefiber 70. FIG. 4 shows the fall-off of the zeroth order Stokes signal at1069 nm and the growth of 1178 nm via the Raman conversion process as afunction of position in the Raman fiber for the case of 9 meters of20/400 fiber pumped with 59.3 W of 1069 nm and seeded with 3 W of 1178nm. Any unconverted zeroth-order Stokes signal at 1069 nm will bereflected back into the Raman fiber 70 by the high-reflector Bragggrating 66 tuned to the zeroth-order Stokes line. Because high powerlevels of 1121 nm are obtained in the Raman resonator cavity, leakagethrough the Bragg grating 67 back toward the ytterbium-doped rare-earthamplifier 65 will most likely occur. To prevent this light from enteringthe amplifier 65, a long period or tilted Bragg grating 69 is used todivert the 1121 nm into the cladding, where it will be absorbed. FIG. 5shows the power levels for the circulating first Stokes in theright-propagating and left-propagating directions. The second-orderStokes seed at 1178 nm will pass through the amplifier system withminimal reflections into the Raman resonator cavity tuned to 1121 nmwhere it will be amplified to high power and emerge from the amplifier72. As before, because of potentially low seed powers at the desiredoutput wavelength, it may be necessary to have multiple rare-earth-dopedamplifier/Raman resonator configurations in series to enableamplification of the desired output wavelength to the power levelsdesired. The embodiment of the 1178 system is compatible with alltechniques to suppress SBS to include large mode area fiber, multipletemperature zones, strain, acoustically tailored fiber, multiple typesof fiber, etc. It is also compatible with polarized and unpolarizedmodes of operation as well as continuous wave or pulsed operation on theorder of a microsecond. The cavity buildup time has been for this systemhas been calculated to be 2.5 microseconds.

To prevent significant linewidth broadening of the 10 MHz 1178 nm seed,one of the 1121 nm Bragg gratings 68 needs to be offset so thatfour-wave mixing is impeded and the bandwidth that resonates isnarrowed. For example, a cavity defined by a pair of Bragg gratings 5 cmin length with a B of 10, can achieve a grating offset of 0.02 nm with a2° C. temperature difference between the gratings. Here B=4n δn η L/λ,where n is the mean refractive index of the fiber core, δn is themodulation of the refractive index, η is the fraction of light in thecore, L is the grating length, and λ is the phase-matched wavelength.Such a cavity will have a high reflectivity over only a few GHz. Thetransmission of the two offset gratings is shown in FIG. 6. This willresult in less broadening of the 1178 nm when it is amplified. Also,high dispersion Raman fiber can also be used to break the phase-matchingcondition associated with four-wave mixing. In summary, factors enablinga narrow linewidth output at 1178 nm are seeding, offset of Bragggratings in the 1121 nm cavity, and usage of a high-dispersion Ramanfiber.

The required power level for the 1069 nm zeroth-order Stokes seed signal60 depends on the seed level required within the amplifier 65 for theoutput power desired. Power levels associated with the 1121 nm in theresonator must be considered when designing the system due to powerlimitations associated with the Bragg gratings. Parameters which affectthe power of 1121 nm in the resonator include: 1069 nm input power levelinto the Raman resonator; 1178 nm seed power level; and the length ofthe Raman fiber. In addition, the onset of SBS must be considered fornarrow linewidth applications such as this one. As the input power levelof 1069 nm into the Raman resonator increases, the 1121 nm circulatingpower in the resonator, the output power of 1178 nm, and the SBSincrease, FIG. 7. As the input power level for the 1178 nm seedincreases, the 1121 nm circulating power decreases, the 1178 nm outputpower increases, and the SBS increases, FIG. 8. Also, as the length ofthe 1121 nm resonator increases, the 1121 nm circulating powerdecreases, the 1178 nm output power increases, and the SBS increases,FIG. 9. Generally, SBS mitigation measures will enable an increased 1178nm seed level or an increased resonator cavity length, a higher outputpower of 1178 nm, and a decreased 1121 nm circulating power in theresonator.

The factors enabling a high output power level of narrow linewidth 1178nm in this invention are the generation of very high levels of the firstorder Stokes line in the 1121 nm Raman resonator, in addition to thefact that the system is seeded. This high power occurs for two reasons,first, high power levels of the 1069 nm are directly injected into thesystem and second, because the 1121 nm cavity is defined by highreflector Bragg gratings, the power level is able to build up to highlevels. High power levels of 1121 nm in addition to seeding at 1178 nmleads to good conversion from 1121 to 1178 nm in a short Raman fiber,several meters in length. In addition, the short Raman fiber enables ahigh SBS threshold which enables a higher output power level of 1178 nm.Because high output power levels of 1178 nm can be obtained from oneamplifier, coherent combination of two or more amplifier chains is notnecessary. The result is a simpler system.

Remote Sensing Laser System—Nested Raman Resonator

Another special case of the above embodiments are systems which generate1240 nm for remote sensing of the water content of the earth and otherplanets. A block diagram for the system is shown in FIG. 10. The firstdevice in this diagram is a third-order Stokes signal 80 tuned to 1240nm having the desired output linewidth. The system is co-seeded with azeroth-order Stokes signal 81 at 1066 nm. The linewidth of this shouldbe broad enough to avoid problems with SBS, but narrower or the same asthe linewidth of the desired 1240 nm output. The third-order Stokessignal at 1240 nm is passed through an isolator 82 and into a WDM 84.The zeroth-order Stokes signal at 1066 nm is passed through an isolator83 and into a WDM 84. The WDM 84 places both the zeroth and thethird-order Stokes signals into the core of the fiber. Both the zerothand third-order Stokes signals enter a ytterbium-doped rare-earthamplifier 85 where the zeroth-order Stokes signal is amplified. Theamplified zeroth-order Stokes signal at 1066 nm along with thethird-order Stokes seed at 1240 nm pass through two long-period ortilted-Bragg gratings 91 and 92 tuned to 1176 and 1118 nm, respectively,and enter the Raman resonator configuration. Upon entering the 1118 nmRaman resonator defined by the highly reflective Bragg gratings 88 and89, the 1066 nm zeroth order Stokes signal is converted to the firstorder Stokes line at 1118 nm by the Raman process in silica orgermanosilicate Raman fiber 93. Any unconverted zeroth-order Stokessignal at 1066 nm will be reflected back into the Raman fiber 93 by thehigh reflector Bragg grating 86 tuned to the zeroth order Stokes line at1066 nm. Because high power levels of 1118 nm are obtained in the Ramanresonator cavity, leakage through the Bragg grating 88 back toward theytterbium-doped rare-earth amplifier will most likely occur. To preventthis light from entering the amplifier 85, a long-period or tilted Bragggrating is used to divert the 1118 nm into the cladding where it will beabsorbed. The first-order Stokes line at 1118 nm is then converted tothe second-order Stokes line at 1176 nm. Any unconverted 1118 nmfirst-order Stokes signal is redirected back into the Raman fiber 93 bythe highly reflective Bragg grating 89. Also, any 1176 nm second-orderStokes signal that leaks out of the Raman resonator cavity through theBragg grating 87 will be directed by the long-period or tilted Bragggrating 91 into the cladding where it will be absorbed. The third-orderStokes seed at 1240 nm will pass through the amplifier system into theRaman resonator cavity tuned to the second-order Stokes line at 1176 nmwhere it will be amplified to high power and output from the amplifier96. SBS needs to be considered for the 1240 nm only if it is narrowlinewidth. Once again, because of potentially low seed powers at thedesired output wavelength, it may be necessary to have multiple, i.e.,1, 2, . . . n, rare-earth-doped amplifier/Raman resonator configurationsin series to enable amplification of the desired output wavelength tothe power levels desired. The embodiment shown in FIG. 10 is compatiblewith all techniques to suppress SBS to include large mode area fiber,strain, multiple temperature zones, acoustically tailored fiber,multiple types of fiber, etc. The embodiment is also compatible withpolarized and unpolarized modes of operation as well as continuous waveor pulsed operation.

To prevent significant linewidth broadening of the 1240 nm seed, one ofthe Bragg gratings 89 and 90 in each of the 1118 and 1176 nm resonatorsneeds to be offset so that four-wave mixing is impeded and the bandwidththat resonates is narrowed. Offset of the Bragg gratings can beaccomplished using a grating controller 95 to either stretch, heat, orbend the gratings. Finally, a high-dispersion Raman fiber can also beused to break the phase matching condition associated with four-wavemixing.

Remote Sensing Laser System—Unnested Raman Resonator

Another embodiment of the system involves replacing the Raman resonatorsshown in the dashed box in FIG. 10 with the configuration shown in FIG.11 where the Raman resonators are completely unnested. In thisembodiment, the amplified 1066 nm (zeroth-order Stokes signal) 100 andthe seed for the 1240 nm (third-order Stokes line) 101 enter the Ramanresonator configuration. The amplified zeroth-order Stokes signal andthird-order Stokes signal are passed through a long-period grating tunedto the first Stokes line 105 and are injected into the first Ramanresonator in the series. The zeroth-order Stokes signal passes through ahigh reflector Bragg grating tuned to the first-order Stokes line 103into the germanosilicate Raman fiber 110. Any unused zeroth-order Stokessignal at 1066 nm is reflected back into the Raman fiber 110 by ahigh-reflector Bragg grating 102 set at the zeroth-order Stokes line. Inthis Raman resonator, defined by the high reflector Bragg grating 103and the output coupler Bragg grating 104, the zeroth-order Stokes signalis converted to the first-order Stokes line at 1118 nm via the Ramanprocess. Any first-order Stokes signal that leaks out of the Bragggrating 103 back toward the rare-earth-doped amplifier will be put inthe cladding by the long period or tilted Bragg grating 105, where itwill be absorbed. The third-order Stokes signal at 1240 nm together withthe first-Stokes signal at 1118 nm, pass through a long-period gratingset at the second-order Stokes line 109 prior to entering the secondRaman resonator defined by the high reflector Bragg gratings 107 and108. In this resonator, the first-order Stokes line is converted to thesecond-order Stokes line in the germanosilicate Raman fiber 111. Anyunused first-order Stokes signal will be reflected back into the Ramanfiber 111 by the high reflector Bragg grating 106. Any second-orderStokes signal which leaks out of the Bragg grating 107 back toward therare-earth amplifier will be put into the cladding by a long period ortilted Bragg grating 109 and absorbed. Upon entering the Raman resonatortuned to 1176 nm, the 1240 nm seed (third-order Stokes signal) isamplified to high levels and output from the system 114. SBS needs to beconsidered for the 1240 nm only if it is narrow linewidth. Again,because of potentially low seed powers at the desired output wavelength,it may be necessary to have multiple rare-earth-doped amplifier/Ramanresonator configurations in series to enable amplification of thedesired output wavelength to the power levels desired. The embodimentshown in FIG. 11 is compatible with all techniques to suppress SBS toinclude different types of fibers, strain, multiple temperature zones,large mode area fiber, acoustically tailored fiber, etc. In addition,the embodiment is compatible with polarized and unpolarized modes ofoperation as well as continuous wave or pulsed operation.

To prevent significant linewidth broadening of the 1240 nm seed, one ofthe Bragg gratings 102 and 108 in each of the 1118 and 1176 nmresonators needs to be offset so that four wave mixing is impeded and anappropriately narrow bandwidth resonates to aid in suppression oflinewidth broadening. Offset of the Bragg gratings can be accomplishedusing grating controllers 112 and 113 to either stretch, heat, or bendthe gratings. High dispersion Raman fiber can also be used to break thephase-matching condition associated with four-wave mixing.

1300-1500 nm Laser System

The final exemplary application for the invention discussed in thispatent is the creation of a series of lasers in the 1300-1500 nmspectral region for telecommunications applications. A block diagramshowing a general embodiment of the system is shown in FIG. 12. Toenable the desired output wavelength of the N^(th) order Stokes line,the following parameters are adjustable: the input zeroth-order Stokeswavelength, the type(s) of Raman fiber used in the system, as well asthe center reflectivity wavelength for the gratings. In the diagram, thefirst device shown is a seed 120 for the N^(th) order Stokes signalhaving a linewidth consistent with what is required of the output.Another device in the diagram is a seed 121 for the zeroth-order Stokessignal at a wavelength appropriate to enable the desired outputwavelength on the N^(th) order Stokes line. The seed for the N^(th)order Stokes line is passed through an isolator 122 and into a WDM 124.The zeroth-order Stokes signal 121 is passed through an isolator 123 andinto the same WDM 124. Both the zeroth and N^(th) order Stokes seedsignals enter a rare-earth-doped amplifier 125 where the zeroth-orderStokes seed is amplified. Both the amplified zeroth-order Stokes signaland the N^(th) order Stokes seed enter multiple Raman resonators 128which may be in various configurations. Within the Raman resonatorconfiguration, the zeroth-order Stokes signal is converted to theN−1^(th) order Stokes signal in steps (zeroth order Stokes signal→firstorder Stokes signal→second order Stokes signal→ . . . →N−1^(th) orderStokes signal) in silica, germanosilicate, and/or phosphosilicatefibers. The seed for the N^(th) order Stokes line is then amplified to ahigh power level by the N−1^(th) order Stokes signal in a single passthrough the system. The output of the system 129 is the N^(th) orderStokes signal at the desired wavelength between 1300 and 1500 nm. Onceagain, because of potentially low seed powers at the desired outputwavelength, it may be necessary to have multiple rare-earth-dopedamplifier/Raman resonator configurations in series to enableamplification of the desired output wavelength to the power levelsdesired.

For telecommunication applications, the ability to create lasers havingsmall shifts in wavelength to enable wavelength division multiplexing ina certain bandwidth is necessary. Small shifts in the output wavelengthare achievable by adjusting the wavelength of the Nth order Stokes line,the wavelength of the zeroth-order Stokes signal and/or the centerwavelength of the Bragg gratings. This embodiment is compatible with alltechniques to suppress SBS to include large mode area fiber, strain,multiple temperature zones, acoustically tailored fiber, etc. Inaddition, the embodiment is compatible with PM as well as non-PM modesof operation in addition to continuous wave and pulsed operation.

The invention claimed is:
 1. A method of generating a high-power lasersignal at 1240 nm having a narrow and controllable linewidth, byemploying a dual-seeded linear Raman amplifier, comprising: a. supplyinga zeroth-order Stokes wavelength signal at 1066 nm; b. supplying athird-order Stokes wavelength seed signal at 1240 nm, at a linewidthapproximately equal to a desired output signal linewidth, and having aseed power level; c. converting the zeroth-order Stokes wavelengthsignal to a first-order Stokes wavelength signal at 1118 nm; d.converting the first-order Stokes signal at 1118 nm into a second-orderStokes wavelength signal at 1176; and e. amplifying the seed power levelthrough Raman amplification using the second-order Stokes wavelengthsignal at 1176 nm, whereby f. a third-order Stokes wavelength lasersignal at 1240 nm having the desired output signal linewidth isgenerated having an amplified power level greater than the seed powerlevel.
 2. The method of generating a high-power laser at 1240 nm asdefined in claim 1, wherein: the step of converting the zeroth-orderStokes wavelength signal to the first-order Stokes wavelength signal at1118 nm is comprised of passing the zeroth-order wavelength signal andthe third-order Stokes wavelength seed signal through a first gratingtuned to a first-order Stokes wavelength at 1118 nm, to obtain a firstgrating output, and then inserting the first grating output into a firstRaman resonator comprised of a first Bragg grating tuned to thefirst-order Stokes wavelength at 1118 nm, a first Raman fiber, a secondBragg grating tuned to a zeroth-order Stokes wavelength at 1066 nm, anda third Bragg grating tuned to the first-order Stokes wavelength at 1118nm, whereby the first-order Stokes wavelength signal at 1118 nm isproduced through Raman conversion of the zeroth-order Stokes wavelengthsignal at 1066 nm, and the first-order Stokes wavelength signal at 1118nm and the third-order Stokes wavelength seed signal at 1240 nm comprisean output of the first Raman resonator.
 3. The method of generating ahigh-power laser at 1240 nm as defined in claim 2, wherein the steps ofconverting the first-order Stokes wavelength signal at 1118 nm into thesecond-order Stokes wavelength signal at 1176 and amplifying the seedpower level by using the second-order Stokes wavelength signal at 1176,are respectively comprised of: inserting the output of the first Ramanresonator into a second grating tuned to a second-order Stokeswavelength at 1176 nm, resulting in a second grating output; andinserting the second grating output into a second Raman resonatorcomprised of a fourth Bragg grating tuned to 1176 um, a second Ramanfiber, a fifth Bragg grating tuned to 11118 nm, and a sixth Bragggrating tined to 1176 nm, whereby the second-order Stokes wavelengthsignal at 1176 nm is produced through Raman conversion of thefirst-order Stokes wavelength signal at 1118 nm, and the seed powerlevel is amplified-to the amplified power level by using thesecond-order Stokes wavelength signal at 1176 nm as the 1240 nmthird-order Stokes seed signal passes through and is output from thesecond Raman resonator.
 4. The method of generating a high-power laserat 1240 nm as defined in claim 3, wherein: the first grating is either along period grating or a tilted Bragg grating; the second grating iseither a long period grating or a tilted Bragg grating; the first,second, fourth, fifth and sixth Bragg gratings are high-reflectorgratings, with each having a reflectivity; and the third Bragg gratinghas a reflectivity lower than the reflectivity of any of thehigh-reflector Bragg gratings, whereby the third Bragg grating is anoutput coupler.
 5. The method of generating a high-power laser at 1240nm as defined in claim 4, wherein: the fourth and sixth Bragg gratingseach having a center wavelength, and further comprising; and tuning thesixth Bragg grating to shift and offset the center wavelength of thesixth Bragg grating relative to the center wavelength of the fourthBragg grating, whereby the second-order Stokes wavelength signal at 1176nm within the second Raman resonator has a resonating bandwidth which iscontrollable and prevented from broadening excessively.
 6. The method ofgenerating a high-power laser at 1240 nm as defined in claim 5, whereineach of the first and second Raman fibers is fabricated from one of agroup of photosensitive materials consisting of germanosilicate andphosphosilicate.
 7. The method of generating a high-power laser at 1240nm as defined in claim 5, wherein the first and second Raman fibers arehigh-dispersion Raman fibers, to breakup a phase-matching conditionwhich, if present, would facilitate undesirable four-wave mixing.
 8. Aseeded linear Raman amplifier for generating a high-power laser at 1240nm with a narrow and controllable linewidth comprising: a. a firstgrating tuned to a first-order Stokes wavelength at 1118 nm, forreceiving a zeroth-order Stokes wavelength signal at 1066 nm and athird-order Stokes wavelength seed signal at 1240 nm at a linewidthapproximately equal to a desired output signal linewidth and at a seedpower level, and for outputting a first grating output; b. a first Ramanresonator comprised of a first Bragg grating timed to the first-orderStokes wavelength at 1118 nm, a first Raman fiber, a second Bragggrating tuned to a zeroth-order Stokes wavelength at 1066 nm, and athird Bragg grating tuned to the first-order Stokes wavelength at 1118nm, for receiving the first grating output and for converting the firstgrating output into a first Raman resonator output comprised of afirst-order Stokes wavelength signal at 1118 nm and the third-orderStokes wavelength seed signal at 1240 nm; c. a second Bragg gratingtuned to a second-order Stokes wavelength at 1176 nm, for coupling thefirst Raman resonator output to a second Raman resonator; d. the secondRaman resonator being comprised of a fourth Bragg grating tuned to 1176nm, a second Raman fiber, a fifth Bragg grating tuned to 1118 nm, and asixth Bragg grating tuned to 0.1176 nm; and e. the second Ramanresonator being for generating a second-order Stokes wavelength signalat 1176 nm and for amplifying and outputting the third-order Stokeswavelength signal at 1240 nm, with the latter having the desired outputsignal linewidth and having an amplified power level greater than theseed power level.
 9. The seeded linear Raman amplifier as defined inclaim 8, wherein: the first grating is either a long period grating or atilted Bragg grating; the second grating is either a long period or atilted Bragg grating; the first, second, fourth, fifth and sixth Bragggratings are high-reflector Bragg gratings, with each having areflectivity; and the third Bragg grating has a reflectivity lower thanthe reflectivity of any of the high-reflector Bragg gratings, wherebythe third Bragg grating is an output coupler.
 10. The seeded linearRaman amplifier as defined in claim 8, wherein: the fourth and sixthBragg gratings each have a center wavelength; and the sixth Bragggrating can be tuned to shift and offset the center wavelength of thesixth Bragg grating relative to the center wavelength of the fourthBragg grating, whereby the second-order Stokes wavelength signal withinthe second Raman resonator has a resonating bandwidth which iscontrollable and prevented from broadening excessively.
 11. The seededlinear Raman amplifier as defined in claim 8, wherein each of the firstand second Raman fibers is fabricated from one of a group ofphotosensitive materials consisting of germanosilicate andphosphosilicate.
 12. The seeded linear Raman amplifier as defined inclaim 8, wherein the first and second Raman fibers are high-dispersionRaman fibers, to breakup a phase-matching condition which, if present,would facilitate undesirable four-wave mixing.